1. Field of the Invention
The present invention relates to a method for generating a color coordinate transformation table that is used for transforming a color signal reflecting the characteristic of a color reading device such as an image scanner into a color signal corresponding to chromaticity coordinates in a different color space, a color coordinate transformation table generation apparatus using the above method, and a storage medium on which a related color coordinate transformation table generation program is recorded.
When an image on an original is read by a color reading device such as an image scanner, a color signal of each pixel may be converted into chromaticity coordinates that represent an equivalent color in a color space that is convenient for image processing by a computer or the like.
In this case, if a color coordinate transformation table having a function of compensating for the input characteristic of the color reading device is prepared as a color coordinate transformation table for transformation from the RGB color space to the L*a*b* color space, chromaticity coordinates representing a device-independent color of each pixel can be obtained by transformation processing using this color coordinate transformation table.
2. Description of the Related Art
International Color Consortium (ICC) has proposed that the device characteristic of a color reading device such as an image scanner or an output device such as a printer should be represented by a profile that indicates a relationship between a device-dependent color space and a device-independent color space.
The profile is an example of the above-mentioned color coordinate transformation table. For example, a profile representing the input characteristic of an image scanner is a collection of about 5,000 pairs of color signals indicating coordinates in the RGB color space and chromaticity coordinates in the L*a*b* color space.
FIG. 17 shows the configuration of an exemplary color coordinate transformation managing device.
In the color coordinate transformation managing device of FIG. 17, an input color transforming section 11 calculates chromaticity coordinates corresponding to a color signal that is input from an image scanner (hereinafter abbreviated as “scanner”) 17 based on a scanner profile that is stored in a scanner profile storing section 12, and stores the obtained chromaticity coordinates in a chromaticity coordinate storing section 13 so that they will be used for processing in an image processing section 14.
On the other hand, an output color transforming section 15 transforms chromaticity coordinates that are stored in the chromaticity coordinate storing section 13 into a color signal using a display profile that is stored in a display profile storing section 16 so that it will be used for display processing in a CRT display device (abbreviated as “CRT” in FIG. 17) 18.
By causing the above color coordinate transformation managing device to perform color transform processing using a profile suitable for an input device and a profile suitable for an output device, colors of an original can be reproduced faithfully on the CRT display device 18, irrespective of the input characteristic of the scanner 17 and the output characteristic of the CRT display device 18.
Conventionally, a scanner profile and a display profile of the above kind are produced according to the following procedure.
For example, to produce a scanner profile, first, the standard color chart that is prescribed by ISO is read by a subject scanner, whereby color signals P1-Pn are obtained each of which indicates intensity of components corresponding to the three primary colors that are included in reflection light coming from a number n of regions on the chart.
Then, sets C1-Cn of chromaticity coordinates indicating device-independent colors of respective regions on the standard color chart are measured by a calorimeter or the like. As shown in FIG. 18, each set of chromaticity coordinates thus measured is correlated with a color signal corresponding to the same region. In this manner, a basic association table as a base of a profile is obtained. The basic association table represents relationships between the color signals P1-Pn and the sets C1-Cn of chromaticity coordinates, respectively.
Then, interpolation processing is performed based on this basic association table, whereby another association table, which represents relationships between color signals locating at uniformly distributed grid points in the RGB color space and sets of chromaticity coordinates of device-independent colors corresponding to the sets of color signals in the L*a*b* color space, is generated. A profile is completed in this manner.
Conventionally, every time a scanner profile is to be produced, an association table having 5000 entries is generated by performing measurements of the above kind on the 288 colors on the standard color chart and performing interpolation processing on a resulting association table of 288 pairs.
Among the techniques relating to the interpolation processing is a technique disclosed in Japanese Patent Laid-Open No. 307872/1995 in which linear interpolation processing is performed on a basic association table using obtained pairs of measured values to generate an association table having a large number of entries.
Another method has been proposed in which a spectral characteristic of a scanner is inferred based on a basic association table of measured values and a relationship as a profile is obtained by using the inferred spectral characteristic.
Among the various techniques of inferring an input characteristic of a color reading device such as a scanner, is a technique disclosed in Japanese Patent Laid-Open No. 46252/1994 in which a spectral characteristic is inferred by solving, using the least-squares method, a simultaneous equation that has been derived from measured values.
However, measuring, with a colorimeter, colors of a lot of color samples as regions of the standard color chart and also reading these color samples with a subject color reading device are still required for producing profiles by performing interpolation on a basic association table, and according to a spectral characteristic inferred based on measurement results.
The above operations require much manpower, and accounts for a large part of a profile producing operation. Besides, these manpower consuming operations are necessary every time a profile is produced.